1. Field of the Invention
The present invention relates to an image processing apparatus for changing a processing method according to an image attribute. More particularly, the present invention relates to an image processing apparatus capable of precisely detecting a character image in a halftone dot image and applying optimal processing to image data based on the detection result.
2. Description of Related Art
In a digital copying machine or the like for reading an image by using a CCD sensor and decomposing the read image into image pixels, thereby carrying out image processing, an optimal image processing method differs depending on a character image, a photography image, and a halftone dot image. Because of this, in such a digital copying machine or the like, the image attribute is discriminated, whereby the image processing method is changed according to the discrimination result. When the halftone dot image is copied, a moire may occur due to interference between the CCD sensor resolution (sampling frequency) and a document halftone dot frequency. If such a moire occurs, an image quality is degraded. Thus, it is required to precisely discriminate a halftone dot region that exists in the document image, and carrying out image processing such that an occurrence of a moire is restrained.
Conventionally, in an image processing apparatus mounted on a digital copying machine, discrimination in the halftone dot region is carried out. As an example of such discrimination, attention is paid to isolation points (maximum and minimum value points with respect to lightness) which is a characteristic of the halftone dot region, the number of isolation points that exist in a predetermined region is counted, and the discrimination in the halftone dot region is carried out according to the count value of such isolation points. In addition, another discrimination is as follows. After a distance between the halftone dot characteristic points has been computed, in the case where the computation result includes periodicity, it is discriminated that the halftone dot region exists. A smoothing process is applied to a region discriminated to be a halftone dot region, thereby restraining an occurrence of a moire.
However, in the above described conventional image processing apparatus, there has been a problem that a character image in a halftone dot image cannot be reproduced precisely. That is, in the reproduced image, the character image in the halftone dot image blurs, and thus, a reproduced image with its high quality cannot be obtained. This is because the smoothing process is carried out for the halftone dot region, and thus, the smoothing process is applied to the character image in the halftone dot image as well. Namely, although it is required to apply processing for enhancing an edge to a character region, the smoothing process which gives priority to the gradation is applied. Therefore, the character image in the halftone dot image blurs.